The second thermostat is a backup thermostat, tested on our own experience, it is advisable to connect a low-power bulb in parallel to the backup thermostat (the one for a high temperature), if the bulb lights up, then you need to change the main thermostat. Your attention is a real electric boiler, under a standard heating element heater 4-6 kW. Heating water for a shower or in the country
heating a private house With your own hands